Metal fabrication



3,510,296 METAL FABRICATION Theodore R. Bergstrom, Little Canada, Minn., assignor to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Original application Aug. 10, 1964, Ser. No. 388,614. Divided and this application Mar. 7, 1967, Ser. No. 644,414

Int. Cl. B22f 3/10; C22c l/05, 33/02 US. Cl. 75-201 2 Claims ABSTRACT OF THE DISCLOSURE This application is a division of 388,614, filed Aug. 10, 1964, now abandoned.

This invention relates to powder metallurgy and more particularly to the production of structures from powdered metals, and more particularly to powdered refractory metals.

The Group VI-B refractory metals as generally available are brittle and must be worked and stress-relieved to produce mill shapes which are of practical utility. Further fabrication including welding, brazing, diffusion bonding, mechanical fastening, on formed sheet or forged, extruded or drawn parts is necessary to produce structures of practical utility. These operations are costly, time consuming and, particularly with respect to rolling, can be carried out only with simple, elongated structures or shapes such as sheets, angles, I-beams and the like. Often, repeated heating is necessary for fabricating even such simple shapes, as the more brittle metals can only be bent or drawn at high temperatures if cracking is to be avoided.

Furthermore, so far as it is known, it has heretofore been believed that in order to obtain these refractory metals in ductile form, essentially all carbon must have been removed from the metal, so that ductile shapes can be produced by mechanical working.

It has now been found that, contrary to what was believed in the prior art, the presence of carbon in certain amounts in refractory metals makes possible the direct production of ductile structures, even of intricate form, without mechanical working, by powder metallurgy processes.

It is an object of the present invention to provide essentially ductile, strong, mechanically unworked structures of refractory metals.

It is a further object of the invention to provide a process whereby green structures can be made which on firing produce ductile refractory metal structures of unworked form, which are suitable as intermediate forms for further mechanical working into sheet, bar, rod, tube and the like.

It is a still further object of the invention to produce various forms of refractory metal structures which are unworked and of an exceedingly fine grain size, and which retain their fine grain structure after heating to very high temperatures.

An additional object of the invention is to produce ex- "United States Patent hoe ceedingly complex structures of very high density without the use of pressure or mechanical working.

Another object of the invention is to produce refractory metal containing carbon in such a manner that the carbon is ideally dispersed therein.

Other objects will become apparent from consideration of the following disclosure.

In accordance with the above-stated and other objects of the invention, it has surprisingly been found that when small amounts of carbon are uniformly distributed throughout a refractory metal (believed to be present as metal carbide and an actual solution of carbon in the metal, i.e., an alloy, as will appear hereinafter), the resulting metallic structure is relatively ductile and strong, even when mechanically unworked. The metal structures thus produced are without preferred orientation of the grains, but on examination of metallurgical samples under the microscope are found to have characteristic metallic grain structure in which metallic carbide is visibly present along the grain boundaries.

It has also been found unexpectedly that the grain size in molybdenum, tungsten and other metallic structures produced by the methods disclosed herein remains appreciably finer after exposure to temperatures in the range of about 1800-2000 C. than does that of structures produced by the conventional working processes.

By the term unworked as used herein, it is meant that the metallic article has not been pressed, rolled or otherwise mechanically worked during or after sintering. The green intermediate structures may, however, be pressed or rolled to some extent as a part of the green fabrication process.

The ductile refractory metallic articles of the invention are produced by a process of powder metallurgy, in which a green structure adapted to firing is first prepared. This green structure comprises the refractory metal in finely divided form and a binder therefor, the binder being a film-forming, essentially non-volatile, organic polymer which thermally degrades on heating to form carbon. A sufficient amount of the binder is employed to coat each particle of the refractory metal over substantially all of its surface.

These green structures can be made in the form of green sheets, which are produced with the aid of a solvent for the binder so as to produce initially a slurry, thick paste or plastic mass of metallic powder, binder, plasticizer and solvent. Even almost dry powder-binder aggregates can be used. This green mass is shaped by rolling, pressing, extruding, knife-coating, etc., evaporating the solvent if necessary, to form a desired shape. Dry, green shapes so formed are then temporarily adhered to form the configuration desired in the final structure, with proper allowance for shrinkage which occurs during sintering. This temporary adherence is readily brought about by cementing the shaped green articles, e.g., sheets, together with a kind of glue made by adding solvent to a paste of metal and binder until the consistency desired is obtained, coating the contacting surfaces with this paste, and pressing them together. Alternatively, the surfaces to be joined can be softened by wetting with a suitable solvent for the binder and then pressed together. If a film of thermoplastic material is employed as a binder, heat seals can be made.

After the desired configuration of green sheets and other forms has been made, it is carefully dried to remove practically all of the solvent and pre-cintered to decompose the binder to make carbon available for oxide reduction, formation of carbides and/or dispersion in solution in the metal grains, as an alloy. Any gaseous decomposition products escape through the somowhat porous mass. The article thus produced is finally sintered to pro- 3 duce a dense, refractory metal structure corresponding in configuration to the original intermediate green structure. A certain amount of shrinkage takes place, depending upon powder size, green density and required density of the final structure.

Some of the carbon is believed to form an alloy or solid solution, or in some cases metal carbides, but regardless of theory as to the actual site of, or action of carbon, the advantageous and unexpectedly superior structures are produced after sintering. It is further believed that the organic material in addition to bonding, thoroughly coats all metal particles and upon thermal degradation to a carbon residue effectively reduces surface metal oxides by the evolution of carbon monoxide or carbon dioxide and produces the oxide-free surface on the particles that is needed for effective sintering.

By the present method it is possible to produce metallic articles having very high density, being of practically any shape and size, and having uniform density and ductility throughout. The parts of the articles are not welded by the usual processes or soldered or brazed using different metals; instead the joints are sinter-welded and are of the same strength, density and ductility as the remainder of the article.

The metals which can be used in the process of the invention to produce the articles of the invention include molybdenum, tungsten, chromium, cobalt, titanium and niobium, and alloys thereof. For the process of the invention they are commonly employed in very finely divided form, i.e., 325 mesh or smaller, ranging down to submicron size. However, if desired, larger particles can be employed, and if special results are desired, variously sized metal powders can be used in combination.

The binders which are employed for making the green structures include those which will form films, which are t organic in nature and which will decompose on heating to form carbon. To these may be added plasticizers to make the sheets which are produced more flexible, or solvents for reducing viscosity. Exemplary binders are methyl cellulose, ethyl cellulose, polyvinyl alcohol, cellulose acetate, phenolformaldehyde resins, urea-aldehyde resins, etc. If nonvolatile plasticizers are used, they will on decomposing when heated contribute some of the required carbon.

Suitable solvents include water, a-cetones, lower alcohols, fiuorinated solvents, etc. The exact nature of the solvent is ordinarily without significance, save that it should be practically inert toward the powdered metal which is employed, and preferably is reasonably volatile. Aqueous solvents are preferred.

When resins are used which polymerize in recognizable stages, as for example phenol-aldehyde resins or polyimides, they are usefully employed in their most soluble stages for preparing the green articles. Thereafter, polymerization may continue through the insoluble stages to form relatively rigid objects suitable for firing.

Exemplary plasticizers are such well-known materials as glycerin, or synthetic waxes such as low-molecular weight polyalkylene oxides, etc., used when aqueous solvents are employed; other waxes, e.g., hydrocarbon or vegetable waxes, used when organic solvents are employed; and the like. It will be apparent that volatile plasticizers can be used in addition to such materials as may decompose to form carbon on heating.

The amount of binder employed is chosen to be that amount which will yield the desired quantity of carbon upon heating. Empirical methods can be used to determine the best proportions of binder used for each set of conditions, including firing conditions; and thereafter, the same quantity of binder will yield the same amount of carbon under those conditions. If somewhat too much binder is used, the excess carbon can in many cases be removed by firing to higher temperatures, or by firing in wet hydrogen. If in particular instances this amount of binder does not give a useful flexible green sheet, a fugi- 4 tive binder, for example poly-butene, polyvinyl-butyl ether or the like, can be added in amount sufficient to render the film self-supporting. From about 1 /2 to 10 percent or more of the decomposable binder can be employed, based upon the weight of the metal; preferable, 1 /2 to 5 percent of binder is used.

It is common practice to produce articles from compacts made from metal powders using dies and presses. This type of processing limits the configurations that can be produced and requires expensive tools. Excessive pressures are required to achieve compacts that will sinter to desired final densities. It has been discovered that the above-mentioned disadvantages can be overcome by the process of the invention.

The following examples, in which all parts are by weight unless otherwise specified, will further and more specifically illustrate the particulars of the invention and the process for their production.

EXAMPLE 1 To a mixture of 180 grams of commercially available molybdenum powder, 1.4 micron Fisher number (maximum metallic impurities .015% with 5.76 parts of methyl cellulose (Methocel 60 H.G.) were added 20 parts of 10% glycerin-water solution. Using a sigma blade mixer, the ingredients were thoroughly mixed to a clay-like mass. This mixture was then milled on a rubber mill to a sheet of approximately 50 mils thickness. The sheet was dried overnight at 66 C.

After drying, the green sheet was cut into a number of pieces suitable for testing and then sintered for about 24 hours at 1100 C. in dry hydrogen, followed by 1 hour at 1800 C. in a vacuum furnace at a pressure of approximately 1x10 in Hg. When tested on an In stron tester, using a rate of loading of 0.02 inch/min, gauge width of 0.25 inch and gauge length 1 inch, the following results were obtained with the sintered sheet.

TABLE I Ultimate Tensile 0.2%

tensile offset yield Elongation, Specimens strength, p.s.i. strength, p.s.i. percent 1 Specimen 3 was held at 1650 0. at approximately 1 1O' mm. Hg for 1 hour after its initial sintering at 1800 C.

Specimens made in this way could be bent to an angle of approximately over a 1.4T radius at the rate of 0.02 inch/min. without failure. (T=thickness of sheet being tested.)

Similar specimens were made, where the green sheets were densified by pressing prior to firing. The green sheets were pressed at 30,000 p.s.i. at room temperature, whereupon a thickness reduction of approximately 10% took place. The pressed sheets were then held for 40 hours at 500 C. in dry hydrogen and then fired for 1 hour at 1800 C. at approximately 1 1O- mm. Hg. Upon test ing as set forth above the following values were obtained.

TABLE II Ultimate tensile Elongation, Specimens strength, p.s.i. percent Specimens made in this way could likewise be bent without breaking.

All of the specimens prepared and set forth above had carbon content of about 0.37% by weight on analysis.

When the above process was repeated, using a fugitive binder, comparable samples were obtained, except that they contained no analytically demonstrable carbon. Thus, for example, the procedure was repeated, using polymethyl methacrylate as the binder and nitroethane as the solvent, and an identical sintering cycle was followed. The

EXAMPLE 2 Green sheet material of identical composition to that set forth in Example 1 was pre-sintered in hydrogen for 12 hours at 1100 C. The resulting molybdenum sheets were then sintered as follows:

TAB LE III Green compac- Green sintering time Density Percent tion pressure, density, temp. and after increase p.s.i. g-mJcc. atmosphere sintering in density 30,000 6.71 1 hour, 1,800 C. 9.13 36 vacuum None 4.37 do 7. 98 82. 7 o 4. 56 1 hour, 1,700 O. 8. 15 78.6

vacuum.

It can be seen that exceptional densities are achieved with this relatively coarse powder.

EXAMPLE 3 To a mixture of 1500 grams of commercially available tungsten (1.45 micron Fisher number, maximum metallic impurities 0.0314%) with 30 grams of methyl cellulose (Methocel 60 H.G.) were added 190 cc. of glycerin-water solution. Using a sigma blade mixer, the ingredients were thoroughly mixed to a clay-like mass. This mixture was then milled on a rubber mill to form a sheet of approximately 50 mils thickness. The sheet was dried overnight at 66 C. and then was pressed between flat plates at about 37,000 p.s.i. Specimens were cut from the sheet and sintered for 3 hours at 1300 C. in vacuum (20 microns). A number of these specimens were further sintered for 1 hour at 2000 C. at 4 10- mm. Hg. Strong, dense sheets of tungsten were obtained.

The sheets 'made by the process disclosed herein are significantly stronger after heating than the similarly exposed commercial material. This is amply demonstrated by the test results set forth in the following table. These were found after heating specimens of commercial tungsten sheet and comparable specimens made according to thisexample. All speciments were about /8 inch Wide and 40 mils in thickness and were first exposed to 2000 C. for 1 hour in vacuum prior to testing, then cooled to room temperature and then heated to test temperature as shown. Specimens designated A were commercial sheets; those designated B were made according to this example.

TABLE IV Test Modulus of Specimen temperature, C. rupture, p.s.i.

1 Average of 2.

(Specimens were subjected to bending using a 1 inch span and a load rate of .02 inch per minute. The series of temperatures used bracketed the brittle-ductile transition temperature of the specimens.)

EXAMPLE 4 To a mixture of 4000 grams of tungsten powder (.85 micron Fisher sub-sieve size, ASTMB-330-58T; commercially available; maximum metallic impurities .0142 percent) with grams of methyl cellulose (Methocel 60 H6.) were added 400 cc. of 10% glycerin-water solution. Using a sigma blade mixer, the ingredients were thoroughly mixed to a clay-like mass. A portion of this mix was milled into a sheet approximately 50 mils thick. This green sheet was dried overnight at 66 C. in air and then pressed at 30,000 p.s.i.

After sintering by heating to 2000 C. over a period of 2 hours in vacuo (5 10 mm. Hg) and holding at that temperature for 1 hour, dense, strong sheets of tungsten were produced. Compacts were prepared from the same .powder using 30,000, 50,000 and 100,000 p.s.i. and no binder. These compacts were sintered in an identical 'manner to that of the green sheets. The above specimens as well as specimens of commercial tungsten sheet were heated over a period of 2 hours to 2000 C. under reduced pressure (less than 5X 10 mm. Hg) and held at that temperature and pressure for 1 hour. A grain size comparison revealed that the material produced according to the process of the invention has much finer grain size than the commercial sheet or the compacts made without binder. Specimens of the sheets of the invention, the compacts made from powder and commercial tungsten sheet were again exposed to 2000 C. under the conditions described for 1 hour to determine the effect on grain size and density. The metallic sheets of the invention remained of exceedingly fine grain size, whereas the commercial material and the powder compacts made without binder changes to markedly coarse grain size.

The densities of sintered specimens produced with and without binder were as follows:

TABLE V sintering Pressing Percent pressure, Time, Temp., Density, theoretip.s.i. hrs. C. gm./cc. cal

Powder only 30, 000 1 2, 000 17. 74 91. 8 D 50, 000 1 2, 000 18. 07 93. 6 Do 0, 000 1 2, 000 18. 29 94. 9 Powder and binder 30, 000 1 2, 000 18. 71 96. 9 Powder only 000 2 2, 000 17. 98 93. 1 Do 50, 000 2 2, 000 18. 33 94. 9 Do 100, 000 2 2, 000 18. 62 96. 8 Powder and binder-.. 80, 000 2 2, 000 18.89 97. 9

EXAMPLE 5 A portion of the clay-like mass produced by Example 4 was placed in an extrusion press. A splitter die was used and the material was extruded into a tube of approximately 2.4 inches OD. and 0.100 inch wall.

This green tube was dried in the extruded position and cut into two lengths about 10 inches long and sintered (usnig graphite 'mandrels of 1.665" diameter) at 2000 C. for 1 hour. It is necessary when sintering to heat slowly from 180 C. to 350 C. to permit the binder and plasticizer products to evolve without rupture of the green mass. Seamless tungsten having high density and strength similar to that described above are obtained.

EXAMPLE 6 A clay-like mass of tungsten, methyl cellulose, glycerin and water produced as described in Example 4 was formed into a sheet on a rubber mill, using a roll-roll speed ratio of 1.4 to 1. This sheet containing virtually all of the plasticizer, binder and solvent is extremely flexible and was cut into a number of smaller square sheets measuring several inches along each side. These were corrugated (7 nodes per inch) on a steam heated roll corrugator. The corrugated sheets were allowed to dry and then the nodes were moistened using a 1% methyl cellulose-water solution, and the sheets were stacked on each other, corrugations being joined node to node, the directions of the corrugations of alternate sheets being at right angles. The green, temporarily adhered intermediate structure was fired by the sintering process described in Example 3. An integral article consisting of corrugated tungsten sheets sinter-welded together with no evident in- 7 homogeneity at the welds on microscopic examination of metallurgical specimens was produced.

Similar shapes are made using machined dies to press the corrugations. Die pressing has been found to be an excellent means of producing corrugations of high dimensional tolerance.

EXAMPLE 7 A mixture of molybdenum powder, methyl cellulose and water made by the process of Example 1 was milled on a rubber mill with a roll speed ratio of 1.4 to 1 into a flexible green sheet of 0.040 inch thickness. This sheet was corrugated and fabricated into shapes of various configurations by the technique described in Example 6. Green material made in the identical manner was extruded into tubes and injection or compression molded into various dies.

It is not necessary to mill the mass of green material if it is to be extruded or compression-molded. The integrity of the shape or extrusion is aided, however, if the green mass is held under pressure of about 1 10 or less for a period sufiicient to remove occluded gases prior to extruding or molding. After forming into the desired configuration, the green structure is carefully sintered first at 1100 C. in dry hydrogen or in vacuo, then at 1800 C. in vacuo, as set forth in Example 1. Strong molybdenum articles are formed. Where these are composed of several pieces joined together, they are found to be sinter-welded at the joints, and no discontinuity or inhomogeneity of the metal at these joints can be detected.

What is claimed is:

1. A method for forming articles from refractory metal powders selected from the group consisting of tungsten, molybdenum, and alloys thereof, which comprises forming a substantially homogeneous mixture of powdered metal with a sufficient amount of an organic polymer binder which degrades thermally on heating to form carbon so that after firing, from about 0.05 to 0.80 percent of carbon remains ideally dispersed in the fired article, forming a green structure from the powdered metal and binder mixture, and firing the green structure to sinter the metal particles and convert the binder to carbon which remains in the final article.

2. A process for the production of ductile, unworked articles from hard, refractory metal selected from the group consisting of tungsten, molybdenum, and alloys thereof powders, which comprises mixing a refractory metal powder having particle size of the order of 325 mesh to sub-micron size with from about 1.5 to 10 percent of an organic film-forming binder which decomposes upon heating to give carbon, shaping the mixture of metal powder and binder into a green structure of desired configuration, and firing the stabilized green structure to a temperature sufiicient to convert essentially all of the binder to carbon, and to sinter the metallic particles into a compact dense article of predetermined configuration containing a predetermined amount of uniformly dispersed carbon.

References Cited UNITED STATES PATENTS 2,698,892 l/l955 Hardin 204 X 2,799,912 7/1957 Greger 75-20l X 3,214,270 10/1965 Valyi 7520l CARL D. QUARFORTH, Primary Examiner ARTHUR J. STEINER, Assistant Examiner U.S. Cl. X.R. 75-222 

